a) Field of the Invention
This invention relates to a method for the production of a magnetic disk substrate which is such a high-capacity type as to use an MR head, comprises glass of high quality.
b) Description of the Related Art
Many magnetic disk substrates produced by performing bed nickel/phosphorus plating on the surface of an aluminum alloy blank have been used for the magnetic recording media of computers. Glass substrates have come to be used for small-sized note-type or mobile-type personal computers. Unlike in the case of an aluminum substrate, a magnetic film can be directly applied to the glass substrate without performing bed plating. However, studies have been made on a method in which like in the case of an aluminum substrate, nickel/phosphorus plating is performed on the glass substrate and then a magnetic film is formed on the plated glass substrate to highly densify the memory capacity. In this case, the use of the glass substrate in place of the aluminum substrate reduces the production of a head snap, that is caving in the substrate, caused by the impact of the head as contrasted with the case of using the aluminum substrate.
Generally, when non-electrolyte plating is performed on a non-metal, oxide or plastic, the following method is adopted as shown in, for instance, Japanese Patent Application Laid-Open (JP-A) Nos. S53-19932 and S48-85614. Specifically, the surface of the above material is subjected to pretreatment for plating in which, for example, Sn is absorbed to the surface and a film of a metal (e.g., Pd, Pt or Au) which is reduced in hydrogen overvoltage and is catalytically active is formed on the Sn. More specifically, pretreatment for plating including degreasing, etching, sensitizing, activating and sensitively intensifying steps is carried out and the pretreated surface is then plated using nickel/phosphorus. The plated substrate is polished using well-known polishing particles, e.g., alumina, to obtain a magnetic disk substrate. The mirror-finished surface of the substrate is coated with a magnetic film by sputtering and the resulting substrate is used as a magnetic recording medium. However, when non-electrolyte nickel/phosphorus plating is performed on the surface of glass, the adhesion of the plating is inferior to the case of plating on an aluminum substrate. Hence there is the case where the method is adopted in which the surface roughness Ra of the glass substrate is designed to be in a range between 0.05 and 0.5 .mu.m to thereby increase the adhesive strength. However, after nickel/phosphorus plating is performed on the substrate having such a surface roughness, only usual polishing to a depth of 1 to 3 .mu.m cannot get rid of the roughness of the surface. The roughness left unremoved causes the generation of pits. Also, if the thickness of the left film becomes 5 .mu.m or less by polishing, the polished depth reaches a hollow portion in a plating film which portion is formed at an inferior plating point on the surface of the glass thereby causing polished pits with the result that the glass substrate cannot be used as a substrate for a magnetic recording medium like the above.
In view of this situation, it is an object of the present invention to provide a magnetic disk substrate using, as a plated substrate, glass with high hardness in place of aluminum. The magnetic disk substrate is small-sized, can stand against an impact when it is carried, and avoids leaving struck damages (head snap) even if it is used for recent note- or mobile-type computers having a head which is located as closely as possible to the substrate.